Non-contact, laser-based point and line range sensors are disclosed in U.S. Pat. No. 4,733,969, Laser Probe for Determining Distance; U.S. Pat. No. 4,872,747, Use of Prisms to Obtain Anamorphic Magnification; U.S. Pat. No. 4,891,772, Point and Line Range Sensors; all of the cited patents being owned by the assignee of the present invention.
Laser-based measuring instruments are an improvement over conventional contact measurement devices that use, for instance, ruby-tipped contact probes. Contact probes are incrementally moved toward a surface to be detected by coordinate-measuring machines (CMMs). The precision of CMMs is limited by the diameter of the small ruby sphere attached to the end of the contact probe, and contacting probes cannot be used to measure flexible parts such as thin metal pieces, plastics, liquids, or other soft, deformable materials. Furthermore, contacting probes must be continually retracted and redeployed so that the probe is not dragged along the surface of the piece being measured.
Laser-based measuring instruments are disclosed in the above-referenced patents. The systems disclosed in the referenced patents are an improvement over previous laser-based systems, and use prisms to effect anamorphic magnification of the reflected light beam. The use of anamorphic magnification provides a substantially more compact system for a given standoff distance, while maintaining substantially higher light levels at the light sensor than was obtainable with previous, non-anamorphic systems. Reflected light intensity in laser-based measurements systems is a function of the surface reflectivity of the object being measured, and can vary over a wide dynamic range, on the order of 1,000,000:1. The system referenced in U.S. Pat. No. 4,891,772 employed optical RAMs as light sensors, and included software based exposure control to compensate for the wide range of light intensity to which the optical RAMs were exposed.
The correctness of the exposure in previous optical RAM systems could be judged only after the exposure had been terminated. This necessitated a trial-and-error approach to exposure control, the exposure time being adjusted on successive trials until a satisfactory exposure had been achieved (bracketing). The time required to take such successive exposures slowed the overall operation of the measurement system.
The problems posed by trial-and-error approaches to exposure control were exacerbated when the measurement system was mounted on a mechanical scanning device meant to be operated in a continuous motion mode. Because the time required to obtain a correct exposure reading was unknown in advance, the scanning device needed to be stopped until the series of bracketing exposures was complete. In addition to the obvious slowing of the overall operation, mechanical hysteresis effects inherent to the start and stop operations injected error into the determination of the mechanical position of the sensing head.
CCD arrays offer an alternative light sensor to the optical RAMs used in previous laser-based measuring instruments. In fact, the widespread use of CCD arrays (for instance in facsimile machines) has driven the cost of CCD arrays down to levels where the use of CCDs in place of optical RAMs could provide significant cost benefits. CCD arrays have the further advantage of providing a gray-scale image, as opposed to the binary image obtainable from an optical RAM. Gray-scale images provide much more information about the structure of the viewed object, and enable more sophisticated analysis. CCD sensors that have potential for application in laser-based measuring instruments are described in the 1992 Databook, Dalsa Inc., Waterloo, Ontario, Canada, and the 1991 Databook, Loral Fairchild Imaging Sensors, Milpitas Calif. These manuals also have many application notes and other useful information.
While the use of CCD arrays in laser-based metrological instruments offers certain advantages, the dynamic range of CCD arrays is limited. The manufacturer-claimed dynamic range of 5000:1 for CCD arrays is obtainable only under ideal conditions. Even this dynamic range is nowhere near the range needed for many sensing applications, where return light levels can easily vary over a 1,000,000:1 range. It will be appreciated that overexposure of a CCD array leads to clipping, which causes the brightness gradations in the highlights of the image to be lost. Underexposure of a CCD array results in images that are heavily contaminated by noise. The potential advantages provided by CCD arrays cannot be realized without effectively solving the problems presented by the inherent limited dynamic range of CCDs.
A non-contact, light-based measurement device that incorporated the advantages of CCD sensors while overcoming the limitations presented by the limited dynamic range of CCD arrays, and which could at the same time reduce or eliminate the time required to effect control of sensor exposure, would provide decided advantages over known metrological instruments.